Absolute sensors made from a laminated structure comprising two wafers, exist. In these sensors, a pressure difference between the ambient sensed pressure and the pressure of a reference pressure chamber deflects a sidewall of the chamber, and the deflection is converted to an electrical signal representing the pressure difference. Often, the reference pressure chamber is evacuated to eliminate thermal effects of residual gases in the chamber.
In order to maintain the given correlation between the read-out from the sensor, it is of importance that the vacuum inside the sealed cavity is maintained. Tests have, however, disclosed that leakage from the ambience into the sealed cavity often takes place no matter how the layers are bonded. Often, leakage occurs as lateral permeation of molecules through the bonded layers.
The lateral permeation of molecules has the effect that pressure inside the sealed cavity will raise from vacuum towards the ambience pressure. The raise in pressure over time causes a pressure equalisation between the sealed cavity and the sensed pressure. This equalization occurs fastest and has the largest impact on sensors for high-pressure applications.
Such silicon sensors are disclosed in “High pressure sensor based on fusion bonding”, published in “Proceedings of the Sixth International symposium on Semiconductor Wafer bonding; Science, Technology and Applications in San Francisco, Calif. Sep. 2-7, 2001” (reference 1). An absolute sensor is disclosed in “High-pressure silicon sensor with low-cost packaging”, published in “Sensors and Actuators A: Physical 92 (2001) 16-22” (reference 2).